Effect of stocking rate and rainfall on rangeland dynamics and cattle performance in a semi-arid savanna, South Africa.
Abstract
In order to examine the emerging paradigm of non-equilibrium behaviour of plant-livestock relations in semi-arid rangeland, the effect of stocking rate, rainfall and their interaction on changes in botanical composition, primary production and live weight gain per animal and per hectare, was studied in a semi-arid African savanna. Two adjacent trials, with different starting conditions of rangeland (good vs. poor) and each of three stocking rates replicated twice, were established in 1986 and maintained for 10 years. A simple rotational grazing system using Brahman weaners was employed. Although changes in botanical composition were strongly influenced by rainfall variability, with a dramatic compositional shift induced by the 1991-92 drought, stocking rate had an additional effect over time in the paddocks on sloping land, particularly on the site which started in good condition. High rainfall and light grazing promoted tufted perennial grasses (Themeda triandra, Digitaria argyrograpta, Cymbopogon excavatus [C. caesius], Sporobolus ioclados); heavy grazing and low rainfall promoted some annuals and weakly tufted perennial grasses (Urochloa mosambicensis, Sporobolus nitens); while other annuals (Aristida adscensionis, Enneapogon cenchroides) were favoured by heavy grazing and high rainfall. Patterns of compositional change supported a state-and-transition model. Rainfall had the most marked effect on variability in herbaceous production. Long-term heavy grazing on sloping land resulted in a decline in herbaceous production in both trials. The depletion of herbaceous biomass in a paddock when grazed heavily was more pronounced if botanical composition had changed as a result of drought and grazing. Long-term heavy grazing did not reduce cattle performance (gain animal-1 and gain ha-1). However, during drought cattle performance was worse at high stocking rates on poor condition rangeland than on good condition rangeland. Rainfall was a better predictor of cattle performance than herbaceous biomass and accounted for far more of the variance in gain per animal than did stocking rate. Cattle performance had a curvilinear relationship with rainfall, indicating that a rainfall year of 680 mm is optimal for cattle production in this region. The notion that semi-arid African savannas are non-equilibrium systems in which rainfall overrides grazing was contradicted. Stocking rate determined the requirement of supplementary feeding and influenced gain ha-1 on poor condition rangeland during drought years. In addition, herbaceous productivity was linked to herbaceous composition, which was linked to stocking rate. Key implications for management are (i) the inequality of different parts of the landscape in supporting livestock and in their sensitivity to grazing, slopes being more easily degraded than bottomland; and (ii) the pronounced changes that grazing can induce in semiarid savanna during and subsequent to drought stress.